This invention relates to a novel method for rapid cooling of high temperature reaction gases obtained by thermal cracking of hydrocarbons and to a novel apparatus utilizable for practice of said method.
Methods for obtaining hydrocarbon products of a high commercial value such as olefins, e.g., ethylene, propylene and aromatic hydrocarbons, e.g., benzene and toluene, which methods comprise cooling high temperature reaction gases obtained by thermal cracking of the starting hydrocarbon materials such as naphtha, kerosene and heavy oil and conducting separation and purification of the gases have been known in the art.
In these conventional methods for thermal cracking of hydrocarbons, it is an important technical problem how to cool the resulting high temperature reaction gas rapidly and effectively. The high temperature reaction gas is generally kept at a temperature of 750.degree. to 900.degree. C. and at such high temperature olefins and aromatic hydrocarbons tend to cause undesired secondary reactions. If they are allowed to stand at such high temperature for a long period of time, therefore, they are converted to carbon and heavy oil or methane and hydrogen.
As means for cooling such thermally cracked gases, there has generally been adopted a method in which high pressure water is used as a cooling medium and an indirect heat exchange is conducted between said cooling medium and a high temperature reaction gas.
In this method, however, considerable coking takes place in the heat exchanger during the rapid cooling step and the resulting coke adheres to the inner walls of heat exchanger tubes, thus resulting in clogging of the tubes. Accordingly, the pressure loss in the reaction gas increases in a very short period of time and coincidentally, the pressure inside the thermal cracking apparatus increases. This results in such disadvantages as reduction of yields of olefins. In the conventional rapid cooling method, therefore the operation of the apparatus system should be halted for a while in order to remove mechanically coke formed in the apparatus system. This is a great economical disadvantage.